The absorption mechanism of the water absorbent resin is governed by interactions of the osmotic pressure due to the difference of the electric attraction shown by a charge of polymer electrolyte, the affinity between water and the polymer electrolyte, the molecular expansion due to repulsive power between ions of the polymer electrolyte, and the expansion restraint due to cross-link bonds. Namely, the absorbency of the water absorbent resin depends on said affinity and molecular expansion, and the absorption speed is largely influenced by said osmotic pressure of the water absorbent polymer itself. Therefore, the molecular expansion and the osmotic pressure of the water absorbent polymer depend on the density and distribution of the cross-links introduced therein, or the kind of the cross-linking agent.
As the absorbed amount in the water absorbent resin increases, the stream of absorbed fluid is disturbed due to the adhesion phenomenon between the water absorbent resin particles expanded by the fluid. To improve this, there is a method of obtaining the water absorbent resin particles having hard surface by reacting the surface of the water absorbent resin particles with a cross-linking agent. Such core-shell type water absorbent resin is practically increased in not only the absorbency but also the penetration ratio of the fluid under a certain load, and thus it is possible to prepare the water absorbent resin superior in the absorbency and the absorbency under pressure.
As a solution polymerization method for the internal cross-linking polymerization for preparing the water absorbent resin, the polymerization methods using a belt or a kneader as a reactor have been suggested. In the case of belt-type polymerization, an additional gel fractionation process is needed.
International Publication No. WO2003/51939 discloses a production process for a water absorbent resin, comprising the steps of: carrying out polymerization, involving cross-linking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, wherein the water-soluble unsaturated monomer contains furfural.
U.S. Pat. No. 5,250,640 discloses a method of pulverizing a hydrogel polymer possessing a cross-linked structure into hydrogel particles by giving shear force thereto in a kneader having rotary stirring shafts.
Korea Patent Publication No. 1989-3808 discloses a method of preparing a water absorbent resin characterized in that a hydrated gel polymer being formed during the course of polymerization is fine pulverized by shear force generated by rotation of stirring shafts within a reaction vessel having a plurality of rotary stirring shafts.
Korea Patent Publication No. 2001-62694 discloses a method of preparing an irregular shape basic water absorbent resin, comprising the steps of: reacting a basic resin and a cross-linking agent in order to obtain the basic water absorbent resin, with the process forwarding a cross-linking reaction and pulverizing the resultant product at the same time.
In prior polymerization methods by a kneader, since the gel fractionation starts with the polymerization and the fractionated gel is discharged from the kneader, the size of the gel fractionated with the polymerization in the kneader is uneven and the residence time must be long in order to make the same to the size suitable to be dried, and thus the methods have a problem of that the absorption properties of the gel-type resin deteriorate.